1. Field of the Invention
The present invention is directed to a method for mixing a gas containing nitric oxide (NO) with a gas containing oxygen, as well as to an apparatus for conducting the method.
2. Description of the Prior Art
In recent years, the use of gaseous nitric oxide (NO) has attracted widespread interest in different kinds of therapy. A number of positive effects has been found when small amounts of gaseous NO are administered to a patient via the airways. For example, NO has a relaxing effect on smooth muscle, thereby improving oxygenation and reducing blood pressure across the lung in patients with severe pulmonary disease. More extensive descriptions of the effects conveyed by NO are provided in e.g. PCT application WO 92/10228 and U.S. Pat. No. 5,427,797.
NO, usually diluted with N.sub.2, is supplied in gas cylinders and is subsequently mixed with a respiratory gas, usually a mixture of air and oxygen (O.sub.2), before the final mixture is delivered to the patient. Examples of known administration systems are described in European Application 0 659 445 and Swedish Application 502 724.
The biggest problem with NO is that it is a highly reactive gas and forms, with O.sub.2, nitrogen dioxide (NO.sub.2)--a highly toxic gas even in small concentrations. Since respiratory gas often contains an elevated concentration of 0.sub.2, typically 50-80% O.sub.2, special measures may be necessary to minimize the amount of NO.sub.2 delivered to the patient.
One approach to minimize the opportunity for NO.sub.2 to form is to supply the gas containing NO closely as possible to the patient, even inside the patient. This has the disadvantage, however, that the NO and respiratory gases may not have time to mix thoroughly, and a heterogeneous gas mixture could be carried into the lungs.
Another option is to mix the two gases continuously as they flow at a constant rate past an inspiratory line, so that the patient then draws a fresh mixture into her/his lungs at every breath. Implementing this option is difficult, however, when the patient is incapable of spontaneous breathing with an adequate volume. Moreover, large amounts of gas would be consumed, and gas containing NO would have to be evacuated to prevent a rise in the level of NO.sub.2 in the room.
Another possibility is to mix the gases in the customary fashion and to install an NO.sub.2 absorber or an NO.sub.2 filter before the patient. A disadvantage here is the difficulty in determining the supplied concentration of NO. An absorber must be monitored to keep it from becoming saturated, thereby losing its ability to absorb NO.sub.2, and a filter must be arranged to keep NO.sub.2 from escaping into the room.
Heretofore, the conversion of NO into NO.sub.2 was believed to occur in proportion to the squared concentration of NO times the level of O.sub.2 times time (k.sub.2 *(NO).sup.2 *O.sub.2 *t). In addition, a small amount Of NO.sub.2 may be present in the O.sub.2 --NO mixture. This amount is proportional to the concentration of NO (k.sub.0 *NO).
Recently conducted experiments have disclosed an additional factor. NO was found to have a property which causes initial formation of NO.sub.2 at the instant of mixture with oxygen, which is proportional only to the square of the NO concentration and the O.sub.2 concentration (k.sub.1 *(NO).sup.2 *O.sub.2).
Thus the final conversion equation, designating the concentration of NO.sub.2, is as follows: EQU NO.sub.2 =k.sub.0 *NO+k.sub.1 *(NO).sup.2 *O.sub.2 +k.sub.2 *(NO).sup.2 *O.sub.2 *t